The present invention relates to an improved propulsion system for powerboats. It is well known that the use of a so-called surface-piercing propeller, wherein the propeller runs partially out of the water, will propel a high speed, light weight power boat with greater efficiency, and therefore at speeds higher than those attainable by a propeller which runs fully submerged. However, surface-piercing propellers are known to be inefficient in slow speed operation and they have difficulty in moving a boat onto a plane for high speed operation. Various systems are known which provide for both slow speed, fully submerged, operation and high speed, half-submerged operation of the same propeller. One such system known to the art places a surface-piercing propeller in a tunnel located at the aft underbody of the boat. In such a system, the propeller may run either fully submerged at slow speed, with a considerable increase in thrust, or half submerged, at high speed. Such a system is described in U.S. Pat. No. 3,793,980, issued Feb. 26, 1976.
It is known that in tunnel operation, efficient operation of the propeller is to some degree dependent upon ventilation of the tunnel. Ventilation is known to help to eliminate cavitation and the inefficient operation of the propeller resulting therefrom. Traditionally, however, tunnel ventilation has been achieved by air being admitted to the tunnel from the transom (stern) end of the tunnel. Such a system is described an U.S. Pat. No. 4,371,350, issued Feb. 1, 1983.
It has been found that ventilating air admitted from the stern end of the tunnel is insufficient to fully ventilate the tunnel so that maximum operating efficiency by the propeller may be obtained. This appears to be at least in part the result of the heavy spray, generated by a surface-piercing propeller operating at high speeds, exiting the stern end of the tunnel.